This invention relates to bushings, and in particular to an improved bushing for use in heavy over-the-road vehicles. The invention is particularly directed to bushings which replace lubricated metal bushings for use in class 6, 7 or 8 vehicles having at least six thousand pound axle ratings.
Certain heavy vehicles, such as over-the-road trucks, utilize metal bushings in parts of their suspension systems. One such place is in the eye bushings at the ends of leaf springs of such vehicles, particularly on their front axles, as well as their rear axles. The bushing attaches the spring eye to other parts of the suspension or to the frame of the vehicle. Such bushings are not designed to provide cushioning of shock. They are generally formed of steel, brass or bronze, or some combination thereof. These bushings, however, require regular lubrication, or they will fail. Greasing the bushings requires taking the vehicle out of service and is therefore expensive. The grease itself also creates potential environmental problems. Even with regular maintenance, the bushings wear. Wear of the mating parts of the bushing not only leads to failure of the bushing, but reduction of control of the suspension and of the vehicle. The industry has therefore long sought an acceptable no-maintenance substitute for metal bushings, such as spring-eye bushings, of heavy over-the-road vehicles. Other metal bushings on such vehicles, such as steering link bushings and S-cam bushings for brakes, have similar problems.
Many existing spring eye bushings are held to the frame or chassis of the heavy over-the-road vehicle by removable side bars. These bushings may be installed as a unit by removing one of the side bars. Others, however, are installed in brackets by placing the outer sleeve portion between the arms of the bracket (generally by placing the sleeve in a spring eye and positioning the spring eye between the bracket arms), then inserting the pin through a hole in one of the bracket arms until it is seated in a hole in the other arm. In some such two-part arrangements, the pin is slid into the sleeve; in others, the sleeve and the pin are threaded, and the pin is screwed into the sleeve. Finding a substitute for either of these types of two-part metal bushings by known methods is particularly difficult.
My prior patent, U.S. Pat. No. 4,840,395, solved a long-standing problem with rubber bushings used in heavy vehicles such as trash trucks, off-the-road construction trucks, and the like. In those applications, the elastomeric bushings permit slight conical movement between the parts, cushion shock, permit slight temporary misalignment under loads, and provide freedom from the requirement for regular lubrication. My prior patent solved the long-standing problem of premature failure of rubber bushings by utilizing an improved polymeric elastomer, preferably a polyurethane having a hardness somewhat greater than the nominal hardness of the rubber it replaces, and by making one part of the bushing rotatable with respect to the elastomer while bonding the elastomer to the other part. Attempts to replace metal bushings in over-the-road heavy vehicles with polyurethane bushings, however, have not been successful.
No other acceptable substitutes for lubricated metal spring-eye bushings in heavy, over-the-road vehicles have been found. In some applications, seals may prolong the lubrication interval, but they add expense and complexity and do not eliminate the need for lubrication. More expensive roller bearings have been used and extend the life of the bushings, but they require special fittings and also require lubrication.